Recent XMM-Newton observations of the B2 type star ρ Oph A indicated a periodicity of 1.205 day, which was ascribed to rotational modulation. Since variability of X-ray emission in massive stars is frequently the signature of a magnetic field, we investigated whether the presence of a magnetic field can indeed be invoked to explain the observed X-ray peculiarity. Two FORS 2 spectropolarimetric observations in different rotation phases revealed the presence of a negative (⟨B_z⟩_all = − 419 ± 101 G) and positive (⟨B_z⟩_all = 538 ± 69 G) longitudinal magnetic field, respectively. We estimate a lower limit for the dipole strength as B_d = 1.9 ± 0.2 kG. Our calculations of the Kepler and Alfvén radii imply the presence of a centrifugally supported, magnetically confined plasma around ρ Oph A. The study of the spectral variability indicates a behavior similar to that observed in typical magnetic early-type Bp stars.